Optimization of shaped pulses for radio frequency excitation in NMR logging

The radio frequency (RF) excitation pulse of the nuclear magnetic resonance (NMR) logging tool can realize slice measurement by designing shaped pulses. In the case of a certain main magnetic field, the accuracy of the shaped pulse design has a very important impact on the signal-to-noise ratio (SNR) of the NMR signal and the measurement of the short relaxation signal. Hard pulse excitation will produce an undesirable infinite number of side lobes that may perturb the spins in unwanted regions. Soft pulse can achieve selective excitation and has a better slice profile and shorter energy release time while it is not conducive to the measurement of short relaxation signals. This article focuses on the design of shaped pulses in extreme downhole environments and analyzes the characteristics of the three shaped pulses in the two cases of equivalent bandwidth and equivalent pulse duration. At the same time, a kind of RF-shaped pulse transmitting circuit with phase difference control is realized. According to the pulse type optimization strategy, the appropriate shaped pulse is selected. When echo spacing (TE) >0.6 ms, the SNR can be increased to more than 12%. When TE is small, it will automatically switch to the hard pulse mode, which is good for short relaxation measurement.